Change "Roles and Permissions" to "Roles and permissions"

What does this MR do and why?

On the Settings -> Roles and Permissions page, we have several places where the text Roles and Permissions is shown. As per ~UX feedback, we need to change this to Roles and permissions. This MR lower cases the p in permissions in the following places:

1. In the sidebar item.
2. In the page title.
3. In the breadcrumb.
4. In several places in the documentation.

	Before	After
Roles and permissions list page
Create/edit role page
Role details page

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

For self-managed, follow this video guide (with audio commentary):

2024-09-16_16-27-43

For SaaS, first run these shell commands:

export GITLAB_SIMULATE_SAAS=1
gdk restart

Then follow this video guide (with audio commentary):

2024-09-16_16-30-27

Related to #490138 (closed)